1. Field of the Invention
This invention relates to a source voltage detecting circuit suitable for integrated circuits which is used to monitor the source voltage fed to a microcomputer or the like and to prevent a runaway or malfunction of a microcomputer or the like due to a lowering in said source voltage.
2. Description of Prior Art
An example of a source voltage monitoring circuit which has heretofore been used in common is shown in FIG. 10. In the source voltage monitoring circuit shown in FIG. 10, a reference voltage provided by a Zener diode Z.sub.1, is fed to one of the inputs of a comparator COM.sub.1, and the source voltage V.sub.CC is divided by resistors R.sub.10 and R.sub.11, one such divisional voltage being fed as the detected voltage to the other input of said comparator COM.sub.1. The source voltage drop is constantly monitored in that the reference voltage and the detected voltage is compared with each other by the comparator. In the source voltage monitoring circuit shown in FIG. 10, however, the reference voltage section, the detection voltage section and the comparator section each consume electric current, making it difficult to achieve lower consumption, so that this circuit is not suitable for use in battery-driven portable devices.
Accordingly, a circuit example as shown in FIG. 11 has been put into practical use to keep pace with the recent trend toward lower current consumption. The circuit example shown in FIG. 11 comprises a source voltage drop detecting circuit 11 using transistors Q.sub.2 -Q.sub.3 and resistors R.sub.13 -R.sub.18, a differential amplifier type comparator 12 using transistors Q.sub.4 -Q.sub.8 and resistors R.sub.19 -R.sub.24, and an output control circuit 13 using transistors Q.sub.10 -Q.sub.12 and resistors R.sub.21 -R.sub.24, said source voltage drop detecting circuit 11 being compensated for temperature by V.sub.BE multiplier 14 using a transistor Q.sub.2 and resistors R.sub.13 -R.sub.14. And for hysteresis formation, the difference in V.sub.BE between the transistors Q.sub.6 and Q.sub.7 is utilized, said transistor Q.sub.7 having an emitter area which is N times as large as the transistor Q.sub.6. Further, capacitors C.sub.1 and C.sub.2 are provided for improving the response characteristic.